Monolayer and multilayer coextruded thermoplastic sheet products which provide both gas and moisture barriers and which are thereby useful as containers in the packaging industry are well known. Conventional processes for producing containers from such sheets generally include extruding sheets of plastic material, cutting blanks or billets from such sheets, heating the material to a desired temperature range, and pressure forming the material into containers.
Recently, processes have been developed which form deep drawn containers which are capable of withstanding the pressure and temperature conditions of a retort chamber. See in this regard, U.S. Pat. Nos. 4,836,764; 4,997,691 and 5,091,231 each issued to Keith Parkinson (the entire content of each being expressly incorporated hereinto by reference). In general, the processes disclosed in the Parkinson '764, '691 and '231 patents involves heating a central region of a disc or billet of thermoplastic sheet material to a temperature at or above its melt temperature while simultaneously keeping an annular region surrounding the central melt-phase region at a temperature to maintain the material in its solid phase. Raising the temperature of the central region of the billet so that it is in its melt-phase allows the central region to be deep drawn (i.e., drawn to a ratio of about 3:1 or greater of container lengthwise dimension to diameter (for circular cross-section containers) or average widthwise dimension (for rectangular cross-section containers).
Problems are encountered however if the billets from which such deep drawn containers are made are highly oriented (i.e. having an ASTM D-1204 shrink rate of greater than 20%). For example, if billets formed of highly oriented thermoplastic sheet are employed in the processes disclosed in the Parkinson '764, '691 and '231 patents cited above, there exists the possibility that the billets will shrink during the heating process prior to thermoforming to an extent that the billets can no longer be carried by their respective support structures. As a result, such highly oriented billets can, and do, fall from their respective support structures while being heated in the oven thereby causing problems during production.
It would therefore be desirable if sheets products could be made that had an inherently low orientation (i.e., an ASTM D-1204 shrink rate of less than about 18%) which could then be employed as the source from which billets could be formed for use in the above-cited Parkinson -764, '691 and '231 processes. It is toward fulfilling such a need that the present invention is directed.
Broadly, the present invention is embodied in processes whereby thermoplastic sheets may be made having minimal internal stresses (orientation), and to the resulting relatively low orientation sheets made therefrom. In preferred forms, the present invention is embodied in sheet products having at least one layer formed of a low orientation thermoplastic material. The sheet products of the invention may thus be in the form of monolayer or multilayer sheets. If multilayer sheets are produced, one of the layers may be formed of a thermoplastic material which forms an inherent barrier to oxygen transport therethrough. Such multilayer sheets are therefore particularly useful in the packaging industry--i.e., since containers can be made therefrom which are both retortable and which provide excellent oxygen barrier properties.
In general essence, inherently low orientation sheet is produced by extruding a thermoplastic material in its melt phase onto the surface of a chill roller and thereafter solidifying the sheet at a relatively slow chill rate (e.g., less than about 60.degree. F./min, typically between about 40 to about 60.degree. F./min, most preferably between about 50 to about 55.degree. F./min). Importantly, the just extruded thermoplastic polymeric sheet is urged into continuous surface-to-surface contact with the chill roll by a curtain of air impinging upon the sheet at a velocity sufficient to ensure such continuous surface-to-surface contact between the sheet and the chill roll, but insufficient to impart substantial stress to the sheet. The term "continuous surface-to-surface" contact and like terms, are meant to convey that the sheet-like melt extruded from the die has an entirety of one surface between opposed widthwise points of the melt in contact with a chill roll such that no gaps or spaces exist between the melt and the chill roll along a line joining such widthwise points. "Discontinuous contact" therefore is meant to refer to gaps or spaces that exist between the surfaces of the melt and the chill roll that are present along a line joining opposed widthwise points of the melt.
In such a manner, therefore, the resulting sheet will exhibit a low inherent internal stress level as evidenced by an unusually low shrink rate (ASTM D-1204) of less than about 18%, preferably between about 5% to about 16%, and more preferably between about 5% to about 10%. By way of example, low density polyethylene sheets having shrink rates of between about 15% to about 16% have been successfully produced and employed to form deep drawn containers. The sheets of the present invention are thus especially characterized by such unusually low shrink rates, and hence unusually low inherent internal stress levels, which find particular utility as containers for the packaging industry--especially containers produced by melt-phase deep-draw thermoforming techniques.
These and other aspects and advantages of the present invention will become more clear after careful consideration is given to the detailed description of the preferred exemplary embodiments of this invention which follow.